This invention relates generally to fiber optic accelerometers and particularly to a particle motion accelerometer in which a sensing member has a pair of optical fiber coils formed on opposite sides of a disk arranged to twist in response to acceleration along an axis perpendicular to the plane of the disk.
The common methodology of designing a fiber optic particle motion accelerometer is based on a configuration of a sensing element consisting of two washer shaped unbalanced fiber optic coils that are centrally hard bonded to a washer-like metal plate. The plate of this composite design is mounted in a housing, either around its center to a post or at its perimeter to the outer encasing. In either case the proof mass and the pickoff coils are generally in the same space. Addition of hard mounted weights to the proof mass of the accelerometer will enhance the output of the accelerometer at the cost of reducing its natural frequency. It is well-known that trying to gain output by reducing the sensing element""s natural frequency to approach the band limit, would result in dependence of the accelerometer output on its fundamental mode damping constant, which is a highly volatile parameter. This design in a broadband application requires a large amount of costly fiber, that is formed using a fiber of lower diameter jacket for the same cladding.
The present invention provides an improved accelerometer design based on an understanding of how its different parts contribute to its broadband output characteristics.
A fiber optic particle motion accelerometer according to the present invention comprises a housing with a mid-section mounted within the housing such that the mid-section flexes in response to acceleration along a sensing axis. A proof mass is mounted to an outer edge of the mid-section. A first spiral-wound optical fiber coil is mounted to a first side of the mid-section and a second spiral-wound optical fiber coil is mounted to a second side of the mid-section. A first hinge is formed in the mid-section adjacent the mounting of the mid-section in the housing, and a second hinge is formed in the mid-section between the proof mass and the first and second fiber optic coils.
The fiber optic particle motion accelerometer according to the present invention may further comprise a pair of mounting posts connected to the housing with a central portion of the mid-section being secured between ends of the mounting posts. The first hinge may be defined by a reduced thickness portion of the mid-section between the mounting posts and the fiber optic coils. The second hinge may be defined by a reduced thickness portion of the mid-section, with the first and second hinges having unequal thicknesses.
The fiber optic particle motion accelerometer may also comprise a first spacer mounted between the first side of the mid-section and the first fiber optic coil and a second spacer mounted between the second side of the mid-section and the second fiber optic coil;
The housing may be formed generally as a cylinder having an inner peripheral slot therein with the mid-section having an outer edge portion retained within the inner peripheral slot to mount the mid-section to the housing. The mid-section may have a central passage therein with the proof mass being mounted to an inner edge portion of the mid-section.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.